High voltage current transformer



Oct. 16, 1962 L. w. MARKS HIGH VOLTAGE CURRENT TRANSFORMER Original Filed July 18, 1955 2 'SheetsSheet 1 [rel/67275 Lou/'5 WMar/ra 17m #Zm Oct. 16, 1962 w. MARKS HIGH VOLTAGE CURRENT TRANSFORMER Original Filed July 18, 1955 2 Sheets-Sheet 2 United States Patent 3,059,151 HIGH VOLTAGE CURRENT TRANSFORMER Louis W. Marks, Pittstield, Mass, assignor to General Electric Company, a corporation of New York Original application July 18, 1955, Ser. No. 522,416, now Patent No. 2,947,958, dated Aug. 2, 1960. Divided and this application Feb. 11, 1960, Ser. No. 8,123

11 Claims. (Cl. 317) This invention relates to transformers and more in particular to an improved high voltage current transformer for circuit breaker relaying and metering. This application is a division of my copending application, Serial Number 522,416, filed on July 18, 1955, and titled High Voltage Current Transformer, now United States Patent 2,947,958 which is assigned to the same 'assignee as this invention.

Current transformers are frequently comprised of an annular-shaped primary winding disposed in a dielectric fluid filled tank and having leads extending upwardly through a high voltage bushing to a pair of terminals on an upper terminal assembly. One or more annularshaped secondary windings are interlinked with the primary winding in the transformer tank, the planes of the secondary windings being perpendicular to the plane of the primary Winding. The primary winding is generally comprised of one or several turns of conductors, and the conductors are adapted to carry the heavy power component of the line current without appreciable voltage drop. In transformers of this type, a tubular electrostatic shield surrounding the primary winding for carrying the charging component of the line current extends upwardly through the bushing to surround the primary winding leads. A layer of insulating material is disposed about the electrostatic shield, and a grounding shield surrounds the insulating material and extends upwardly through the bushing to surround the primary winding leads. A layer of insulating material is disposed about the electrostatic shield, and a grounding shield surrounds the insulating material and extends upwardly through a portion of the bushing. The electrostatic shield, the insulating material, and the grounding shield separate the primary winding from the secondary windings. The number of turns in the secondary winding is determined by the particular application of the transformer.

In transformers of this type considerable difliculty is encountered in the winding of the coil inside the electrostatic shield, and the cost of such transformers is thereby increased. A further difiiculty materializes when the transformer is used in circuit breaker applications if an internal electric arc occurs between the primary winding and certain portions of the grounding shield. When used in circuit breaker applications the current transformer is generally connected such that upon the occurrence of any fault that might cause fault currents to circulate in the power lines, the circuit breakers act to disconnect the power source from the section of line having a fault. However, in certain cases, if an internal fault occurs in the transformer there is a possibility that the circuit breakers would not be actuated properly and the current transformer would thus not be isolated from the power lines.

It is therefore an object of this invention to provide an improved high voltage current transformer.

It is also an object of this invention to provide a high voltage current transformer of the eye-bolt type wherein the electrostatic shield serves the function of the primary winding.

Another object of my invention is to provide a simplified construction for the primary winding of a current transformer of the eye-bolt type, wherein the winding "ice is comprised of an open turn of a tubular conductor and the leads joining the turn are comprised of a longitudinally split straight tubular conductor.

A further object of this invention is to provide means in a high voltage current transformer for preventing excessive damage due to internal faults.

A still further object is to provide means in a current transformer for actuating circuit breaker means for isolating the current transformer and adjacent power line sections from power lines in the event of internal failure in the current transformer and also to isolate adjacent power line sections in the event of faults on such adjacent power line sections.

My invention will be better understood from the fol lowing description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a cross-sectional view of an eye-bolt current transformer embodying this invention and illustrating typical connections of the current transformer in circuit breaker applications,

FIG. 2 is a perspective view of the primary winding of the transformer of FIG. 1,

FIG. 3 is a perspective view of a modified form of a portion of the primary winding of FIG. 3,

FIG. 4 is a diagrammatic representation of the current transformer of FIG. 1,

FIG. 5 is a diagrammatic representation of a modified form of the current transformer of FIG. 1,

FIG. 6 is a diagrammatic representation of another modified form of the current transformer of FIG. 1,

FIG. 7 is a diagrammatic representation illustrating the internal fault protection means of my invention as applied to a cable-type current transformer, and

FIG. 8 is a diagrammatic representation illustrating the internal fault protection means of my invention as applied to a twin bushing-type current transformer.

Briefly stated, in accordance with one aspect of my invention, I provide a high voltage current transformer of the eye-bolt type wherein the primary winding is comprised of one turn of a tubular conductor and the primary winding leads connected to the ends of the primary winding and extending upwardly through the transformer bushing are comprised of a longitudinally split tubular conductor. The primary winding also acts as an electrostatic shield and thus carries both charging current and line current. My invention also provides, in a current transformer employing two secondary windings wound on annular magnetic core means for preventing excessive damage to the transformer upon the occurrence of an internal fault from the primary winding to the grounding shield. This protective means is comprised of a grounding cable connected on one end to the grounding shield and passing through both cores so as to couple inductively to the secondary windings and thence through a terminal on the transformer to ground or to a potential device. The grounding shield is provided with a gap in order to prevent the flow of circulating currents in the shield. As will be disclosed in more detail later, the protective circuit means will not function properly unless the grounding cable is affixed to certain portions of the grounding shield with respect to the gap in the shield.

Referring now to the drawings, and more in particular to FIG. 1, therein is illustrated a current transformer 10 having the primary winding 11 and a pair of secondary windings 12 and 13 wound on annular magnetic cores. The windings 11, 12 and 13 are annular-shaped, with the axis of the respective windings 12 and 13 being aligned perpendicular to the axis of the primary winding 11. The two secondary windings and their cores are structurally interlinked with the primary winding, due to the fact that each secondary winding passes through the hollow portion of the primary winding. A pair of leads 14 and 15 extend upwardly from the tank 16 in which the windings are disposed through an insulating bushing 17, and are connected by means of conductors 18 to terminals 19 and 20 respectively positioned on a top terminal assembly 21. The top terminal assembly 21 is a metallic chamber that is electrically connected to the terminal 19 and electrically insulated from the lead 20 by means of an insulating bushing 22. A metallic shield 23 may be provided surrounding the terminal 20 and electrically connected to the top terminal assembly.

The structure of the primary winding may be more clearly seen in FIG. 2 wherein it is shown that the winding 11 is comprised of a single turn of a tubular conductor with the ends of the turn being separated by a short gap 25. The primary winding leads 14 and 15 which are connected to the ends of the primary winding are each onehalf of a longitudinally split tubular conductor. The primary winding assembly may be formed by welding or otherwise electrically connecting a straight tubular conductor to the side of an annular tubular conductor in the plane of the annular conductor, and then splitting the straight tubular section and the annular tubular section along the plane perpendicular to the plane of the annular tubular section and passing through the axis of the straight tube. As illustrated in FIG. 2 it is preferred that the tube from which the primary winding leads 14 and 15 are fabricated has a larger diameter or greater wall thickness than the. tube from which the winding itself is formed, in order that the leads 14 and 15 have the same conductor cross sectional area as the primary winding.

A modification of the primary winding of FIG. 2 is illustrated in FIG. 3. Here a strip 26 of semi-conducting material having sufficiently high resistance to prevent the bypass of appreciable power current therethrough is inserted in the gap between the ends of the primary winding turn and the leads 14 and 15. The semi-conducting material relieves electrostatic stress concentrations that exist in the gaps, and thereby reduces the danger of breakdown in other parts of the transformer. As another alternative the strip 26 may be of a solid insulating material to insure mechanical stability of the primary winding, and a coating of semi-conducting material may be provided on the edges of the strip 26 or a resistance tape may be wound on the leads 14 and 15.

Referring again to FIG. 1 the primary self-shielding winding 11 and primary winding leads 14 and 15 are wrapped in a conventional manner with an insulating material 30. A grounding shield 31 surrounds the primary winding insulation material 30 and extends upwardly through the lower portion of bushing 17. The grounding shield 31 is provided with a gap 32 surrounding the primary winding at one point in order to prevent the flow of circulating current in the grounding shield.

An annular shield member 33 may be provided surrounding the top ends of the primary winding leads 14 and 15 in the bushing 17 or the insulated leads 14 and 15 may be extended into the top terminal assembly 21. The shield '33 is electrically connected to the lead 14.

A grounding cable 35 is electrically connected to the grounding shield adjacent one side of the split 32, passes downwardly through the secondary winding 13 and core, around the bottom of the primary winding, makes a single loop around the secondary winding 12 and core is connected to a terminal 36 on the transformer wall 16. It is to be noted that any current passing through the cable 35 passes through one of the secondary windings in the same direction as normal current in the primary Windings, and passes through the other secondary winding in the direction opposite to the normal current passing through the primary winding 11. Normal current flow in the primary winding may be in either direction.

The current transformer of FIG. 1 is shown connected in a typical circuit breaker application for disconnecting a power line bus from any section having a ground fault and also for isolating the current transformer in the event of an internal fault in the transformer. In this circuit the current transformer 10 is connected in series with a high voltage bus 40. Current may flow in either direction in the bus 40. One end portion 41 of the bus 40 is connected by way of the contacts of a first remote circuit breaker 42 to a second portion 43 of the bus 40 and this second section 43 is connected by way of the contacts of a local circuit breaker 44 to a third portion 45 of the bus 40, which is connected directly to the terminal 19 of the current transformer. The terminal 18 of the current transformer is connected by way of a fourth portion 46 of the bus 40 through the contacts of a second remote circuit breaker 47 to the other end portion 48 of the bus 40. A

remote current transformer 50 on the one end 41 of the bus 40 adjacent the first remote circuit breaker 42 is connected in series opposition with the secondary winding 12 of the current transformer and the differential current from these two source passes through the actuating windings of a local relay 51 and a remote relay 52. The contacts of the remote relay 52 actuate the first remote circuit breaker 42 by conventional means. The winding of another current transformer 53 on the other end portion 48 of the bus 40 adjacent to the remote circuit breaker 47 is connected in series opposition with the secondary winding 13 of the current transformer 10 and the differential current from these two sources passes through the windings of remote relay 54 and the local relay 55. The contacts of remote relay 54 actuate the second remote circuit breaker 47 by conventional means. The contacts of local relays 51 and 55 are connected to separate coils 56 and 57 respectively of a conventional circuit breaker trip mechanism for actuating the local circuit breaker 44 so that closing of the contacts of either or both of the relays 51 and 55 opens the local circuit breaker 44.

The terminal 36 may be connected to ground by way of a switch 58, in which case current passing through the grounding cable in the event of internal failure in the current transformer induces current in the secondary windings 1'2 and 13 to open the circuit breakers in a manner to be more fully disclosed in the following paragraphs. As an alternative, the grounding of the ground cable 35 may also be accomplished by arcing across a spark gap 59, in the event of internal failure in the transformer.

In order to more clearly disclose the action of the current transformer 10 upon the occurrence of various fault conditions, the various conditions and their effect are hereinafter described.

Assuming a power how in bus 40 from left to right in FIG. 1, if a ground fault occurs on portion 43 of the bus, fault current flows in the secondary winding of current transformer 50 and no fault current flows in the secondary winding 12 of current transformer 10. This results in the closing of the contacts of relays 51 and 52, due to differential current and therefore the contacts of the remote circuit breaker 42 and the local circuit breaker 44 are opened, thereby isolating the section of line having the ground fault. If the ground fault occurs on line section 46, fault current flows in secondary winding 13 and no fault current flows in the secondary winding of current transformer 53. This results in the closing of the contacts of relays 54 and 55, and therefore the opening of the contacts of local circuit breaker 44 and remote circuit breaker 47, thereby isolating the section of line having the ground fault. If a fault were to occur between line section 45 and ground, an erroneous signal would occur causing circuit breakers 42 and 44 to open their contacts. Therefore the shield 23 has been positioned surrounding the line 45, so that no fault can occur between the line 45 and ground. A ground fault on the shield appears as a ground fault on line section 46.

Similarly, when power flows from right to left along the bus 40 in FIG. '1, a ground fault on line section 46 causes the opening of the contacts of circuit breakers 44 and 47, a ground fault on line section 43 causes the opening of the contacts of circuit breakers 42 and 44, and a fault on line section 45 to the shield 23 causes the opening of the contacts of the circuit breakers 44 and 47.

A fault involving protective equipment is serious in nature in that it may remove the protection from the circuit. Novel features of this transformer protect the system in the event of internal failure of the protective current transformer.

The protective action afforded by the grounding lead 35 may more clearly be understood by reference to FIG. 4, which is a diagrammatic representation of the current transformer of FIG. 1. Breakdown occurring between the leads 14 and and the primary winding 11 to the grounding shield may be in one of four general areas. A fault may occur in the region 65 between the grounding shield above the gap 32 and the lead 15 or the portion of the primary winding 11 adjacent the lead 15, a fault may occur in the region 66 between the primary winding 11 and the grounding shield above the secondary winding 13 and below the gap 32, a fault may occur in the region 67 between the primary winding 11 and the lower portion of the grounding shield, or a fault may occur in the region 68 between the grounding shield above the secondary winding 12 and the lead 14 or the portion of the primary winding adjacent the lead 14.

When the instantaneous direction of current flow is counterclockwise in the primary winding 11, a fault in region 65 results in fault current flowing through the grounding lead inducing a current proportional to the fault current in secondary windings 12 and 13, the current in winding 12 being reverse in respect to the current normally induced therein. If the fault occurs in region 66, fault current flowing through grounding shield 31 and the grounding cable 35 provides a resultant induced current proportional to twice the fault current in secondary winding 13, and no resultant current in secondary winding 12. The same effect is produced by a fault in region 67 and region 68.

When the instantaneous direction of current flow is clockwise in primary winding 11, a fault in region 65 results in no resultant current being induced in secondary winding 13 and a resultant current proportional to twice the fault current being induced in secondary winding 12. If the fault occurs in regions 66, 67, or 68 a resultant current proportional to the fault current is induced in both of the secondary windings, but the current in secondary winding 13 is in the opposite directional to normal flow therein.

As previously stated, secondary windings 12 and 13 are connected in series opposition with secondary windings of current transformers 53 and 50 respectively (FIG. 1). In the case of faults occurring in each of the regions 65, 66, 67 and 68 the currents in secondary windings 12 and 13 are different from the current in the secondary windings of current transformers 53 and 51) respectively, since in the event of current flow from left to right in bus upon the occurrence of such a fault a current proportional to the fault current is induced in the secondary winding of current transformer and no current is induced in the secondary winding of current transformer 53, and in the event of flow of current from right to left in bus 40 upon the occurrence of such a fault a current proportional to fault current is induced in the secondary winding of current transformer 53 and no current is induced in the secondary Winding of current transformer 50. Therefore, in the event of any internal fault in the regions 65, 66, 67 and 68, the contacts of all three circuit breakers are opened.

A differential current between the secondary winding 13 and the secondary winding of current transformer 53 causes isolation of the current transformer 10, while a differential current between secondary winding 12 and the secondary winding of current transformer 50 causes isolation of power line section 43. In view of this, if it is not required that the power line section 43 be isolated in the event of internal failure in the current transformer 10, the loop of the grounding cable 35 around the secondary winding 12 may be omitted.

A modification of the arrangement of FIG. 4 as illustrated in FIG. 5 has the gap 32 in the grounding shield 31 in the lower portion of the grounding shield between the secondary windings 12 and 13. In this modification the four regions of possible internal faults are the region 7%} between the grounding shield 31 above the secondary winding 13 and the lead 15 or the portion of the primary winding 11 adjacent the lead 15, the region 71 between the primary winding 11 and the lower portion of the grounding shield 31 between the secondary winding 13 and the gap 32, the region 72 between the primary winding 11 and the lower portion of the grounding shield 31 between the gap 32 and the secondary winding 12, and the region 73 between the grounding shield 31 above the secondary winding 12 and the lead 14 or the portion of the primary winding 11 adjacent the lead 14.

In this modification, if the instantaneous current flow is counterclockwise in the primary winding 11, a fault in region 70 or 71 results in current proportional to the fault current being induced in the secondary windings 12 and 13, the current induced in winding 12 being in the opposite direction of normal current flow therein, and a fault in regions 72 or 73 results in no current being induced in winding 12 and a current proportional to twice the fault current being induced in winding 13. If the current flow is normally clockwise in primary winding 11, a fault in regions 70 or 71 results in no current being induced in winding 13 and a current proportional to twice the fault current being induced in winding 12, while a fault in regions 72 or 73 results in currents proportional to the fault current being induced in both secondary windings 12 and 13 with the current in winding 13 being in the opposite direction to normal flow therein. It is readily seen that the modification of FIG. 5 functions the same as the current transformer of FIGS. 1 and 4.

In the modification illustrated in FIG. 6, the ground cable 35 is connected to the ground shield 31 above the secondary winding 12, and the cable passes downward through winding 12 downward through the winding 13, and thence to ground. The resulting currents induced in secondary windings 12 and 13 in the event of internal faults are the same in this modification as in the case of the modification of FIGS. 1 and 4.

The protective arrangement of any invention may also be applied to a cable type current transformer, as illustrated in FIG. 7. This type of transformer is comprised basically of a single loop of a cable having a central conductor 75 surrounded by a grounding shield 76. Secondary windings 77 and 78 wound on generally annular-shaped magnetic cores are positioned coaxial with each leg of the loop. The grounding cable 79 is fastened to the grounding shield 76 above secondary winding 77, passes downward through winding 77 and thence downward through winding 78 to ground.

In FIG. 8 the protective arrangement of my invention is illustrated as applied to a pair of bushing type current transformers. This construction may also be used in a transformer in a single tank, and the primary winding may be multiturn. In this modification, the central primary lead 80 of one current transformer is connected in series with the central primary lead 81 of the other transformer. The central lead 80 is surrounded by a ground shield 82 which is in turn surrounded by a secondary winding 8-3, and the central lead of the other transformer is surrounded by a ground shield 84 which is in turn surrounded by a secondary winding 85. A ground cable 86 is connected to the shield 82 above the winding 83, passes downward through the windings 83 and 85, and thence to ground. A conductor 87 is connected between the upper portions of the grounding shields 82 and 84. As an alternative the conductor may be connected between the lower portions of the ground ing shields 82 and 83.

From the foregoing disclosure then, my invention provides means for inducing a current in each of a pair of secondary windings, such as windings 12 and 13, of a current transformer that is of different phase or magnitude that the current flowing through the relatively opposite terminals, such as terminals 19 and 2t) respectively, of the current transformer in the event of internal fault in the transformer, regardless of the direction of normal current flow in the transformer. It will be obvious that other combinations of the position of the grounding shield gap and the ground cable may also be employed and still retain the protective feature. The protective ground cable means may also be employed in the case where the primary Winding has more than one turn. Therefore, my invention provides a current transformer having simplified construction and means for actuating external circuit breaker means in the event of internal failure thereof.

It will be understood, of course, that, while the forms of the invention herein shown and described constitute preferred embodiments of my invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications thereof. It will also be understood that the words used are words of description rather than of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a high voltage current transformer, a primary winding comprised of an open turn of a conductor, a pair of leads connected to the ends of said turn, insulation material surrounding said primary winding and at least a portion of said leads, a grounding shield surrounding said insulation material, a gap in said grounding shield to prevent circulating currents from flowing therein, a grounded cable having one end connected to said grounding shield, at least one secondary winding interlinked with said primary winding turn, said grounded cable passing through said secondary winding between where said one end is connected to said shield and where it is connected to ground.

2. In a high voltage current transformer, a primary winding comprised of an open turn of a conductor, a pair of leads connected to the ends of said turn, insulation material surrounding said turn and at least the adjacent portions of said pair of leads, a tubular grounding shield surrounding said insulation material, a gap in said grounding shield to prevent circulating currents from flowing therein, at least one secondary winding wound on an annular magnetic core and interlinked with said primary winding turn, a grounded cable having one end connected to said grounding shield, means for grounding the other end of said cable, said grounded cable passing through at least one of said secondary windings between where said one end is connected to said shield and where it is connected to ground, said gap and grounded cable being so arranged that current flowing in one of said secondary windings is proportional to the current flowing through one of said leads regardless of the direction of current flow in said transformer under normal operating conditions and current flowing in said one secondary winding is either of a different proportion to or is out of phase with the current flowing through said one lead when an internal fault exists in said transformer.

3. In a high voltage current transformer, a primary winding comprised of an open turn of a tabular conductor, first and second leads connected to the ends of said turn, said leads being comprised of a pair of spaced apart sections of a longitudinally split straight tubular conductor, insulation material surrounding said turn and at least the adjacent portions of said spaced apart leads, a tubular grounding shield surrounding said insulation material a gap in said grounding shield to prevent circulating currents from flowing therein, first and second secondary windings wound on annular magnetic cores and interlinked with said primary winding turn, said first secondary winding linking the portion of said primary winding adjacent said first lead and said second secondary winding linking the portion of said primary winding adjacent said second lead, a grounding cable having one end connected to said grounding shield, means for grounding the other end of said cable, said grounding cable passing through each of said secondary windings, said gap and grounding cable being 50 arranged that current flowing in said first secondary winding is proportional to the current flowing in said second lead and current flowing in said second secondary winding is proportional to the current flowing in said first lead regardless of the direction of current flow in said transformer under normal operating conditions, and current flow in said first secondary winding is either of a different proportion to or is out of phase with the current flow in said second lead and current flow in said second secondary winding is either of a different proportion or is out of phase With the current flow in said first lead when an internal fault exists in said transformer.

4. In a high voltage current transformer of the eyebolt type, a primary winding comprised of an open turn of a tubular conductor, first and second leads connected to the ends of said turn coplanar with said turn and extending upwardly to a pair of terminals, said leads being comprised of a pair of spaced apart sections of a longitudinally split straight tubular conductor, insulation material surrounding said turn and at least the adjacent portions of said leads, a tubular grounding shield surrounding said insulation material, a gap in said grounding shield to prevent the flow of circulating currents therein, first and second secondary windings wound on annular magnetic cores and interlinked with said primary winding turn, said first secondary Winding linking the portion of said primary winding adjacent said first lead and said second secondary winding linking the portion of said primary winding adjacent said second lead, a grounding cable having one end connected to said grounding shield, means grounding the other end of said grounding cable, said grounding cable passing through each of said secondary windings.

5. The current transformer of claim 4 in which said grounding cable is connected to the portion of the grounding shield above said first secondary winding, passes downwardly through said first secondary winding, thence around said primary winding and passing downwardly through said second secondary winding to said grounding means, and said gap is located between said first secondary winding and the connection between said cable and said shield.

6. The current transformer of claim 4 in which said grounding cable is connected to the portion of said grounding shield above said first secondary winding, passes downwardly through said first secondary winding, thence around said primary winding and passing downwardly through said second secondary winding to said grounding means, and said gap is located in the lower portion of said grounding shield between said secondary windings.

7. The current transformer of claim 4 in which said grounding cable is connected to the portion of said grounding shield above said first secondary winding, passes downwardly through said first secondary winding, thence around said primary winding and passing downwardly through said second secondary winding to said grounding means, and said gap is located in the portion of said ground shield above said second secondary winding.

8. In a high voltage current transformer of the cable type, a primary winding comprised of a loop of a conductor, a grounding shield surrounding said loop, a pair of secondary windings surrounding said grounding shield, a grounded cable having one end connected to said grounding shield, means grounding the other end of said grounded cable, said cable being connected to said grounding shield above each of said secondary windings and passing through each of said secondary windings between where said one end is connected to said shield and where it is grounded.

9. In a high voltage current transformer, a pair of conductors connected in series, each of said conductors comprising the primary winding of a separate bushing type transformer, a grounding shield surrounding each of said conductors, a secondary winding surrounding each of said grounding shields, a grounded cable having one end connected to one of said grounding shields above the respective secondary winding, means grounding the other end of said cable, said cable passing through each of said secondary windings between where said one end is connected to said shield and where it is grounded, and a conductor connected between one end of said of said grounding shields and the respective end of said other grounding shield.

10. In a high voltage current transformer, 21. primary winding having first and second leads extending therefrom, a tubular grounding shield surrounding said primary winding and insulated therefrom, a gap in said grounding shield to prevent the flow of circulating currents therein, first and second secondary windings interlinked with said primary winding, a grounding cable having one end connected to said grounding shield, means grounding the other end of said grounding cable, said grounding cable passing through each of said secondary windings, sa-id gap and grounding cable being so arranged that current flowing in said first secondary winding is proportional to current flowing in said second lead and current flowing in said second secondary winding is proportional to the current flowing in said first lead regardless of the direction of current flow in said transformer under normal operating conditions, and current flow in said first secondary winding is of a different proportion or is out of phase with current flow in said second lead and current flow in said second secondary winding is either of a different proportion or out of phase with the current flow in said first lead when an internal fault exists in said transformer.

11. In a high voltage current transformer of the type enclosed in a housing, a primary winding comprised of an open turn of a conductor, an insulation medium in said housing surrounding said primary winding, a grounding shield between said housing and primary winding, ground means including a grounded lead having one end connected to said grounding shield, at least one secondary winding interlinked with said primary winding turn, and said grounded lead passing through said secondary winding between where said one end is connected to said shield and where it is connected to ground.

References Cited in the file of this patent UNITED STATES PATENTS 2,804,577 Roth Aug. 27, 1957 2,929,963 Kaestle Mar. 22, 1960 2,931,951 Wilson Apr. 5, 1960 

